Method of impregnating wood



March 10, 1953 E GARE) 2,631,109

METHOD OF IMPREGNATING WOOD Filed Dec. 8, 1948 FORM BLANK EARTIALLY FABRICATED GEAR BLANK COMPLETELY FABRICATED GEAR BLANK gwuem fob GEORGE E. GARD Patented Mar. 10, 1953 UNITED STATE PATENT or 2,631,109

METHOD OF IMPREGNATING WOOD? George E; Gard; Lancaster, Pa., assignorto Armstrong Cork Compa-ny, Lancaster,; Pa;, a' cor-- poration' of "Pennsylvania Application December 8, 1948, Serial No. 64,151:

'Claim5.. 1.

Thisinvention relates to a method of impregnating wood and is concerned more particularly with a method of impregnating wood withv an impregnant subsequent to removal of water from the wood until its water content is. essentially zero.

In the manufacture of articles of wood, it is frequently desirable. to impregnate the woodv to eliminate dimensional changes which normally occur through theloss or gain of moisture in the wood, and also to preserve and protect the wood against. deterioration. For example, in the manufacture of nonsparking gears, such as are frequently used in the cork grinding industry, wooden gear-teeth are employed. The teeth are mounted in a metal frame and considerable difficultyhas been experienced because of the dimensional instability of the wood, 105s of moisture and concomitant shrinkage causing the teeth to become loose, resulting in excessive wear and In the manufacture of cork composition and the-like by high frequency dielectricheating, it isdesirable to provide a mold in which the cork composition can be confined during heating. Wood is properly suited for this purpose, provided the moisture canbe removed from the wood andreplaced with a material which will remain within the pores of the wood under theconditions encountered in use. It is. essential that-no-substantial checking of the wood grain occur either in preparation or in use. The wood must be subject to no substantial dimensional change in .use since such dimensional change might introduce cracks in themold which would be undesirable from the standpoint of the application of a high frequency voltage. The molds must be. capable of withstanding continued reheating without substantial dimensional change. Since the rate of dielectric heating and voltage insulating value of the mold will vary drastically with any change in the moisture-content-of the wood, it is essential that a constant level of water content be maintained and this should be essentially zero for best operating results.

There are many other industrial applications of wood where permanent resistance to dimensional change or resistance to deterioration or both are important considerations. The present invention is directed to the problem of rendering wood substantially proof against dimensional change-by amethod which is economically prac- 2 ticable, relatively inexpensive. and operable without. deleterious effecton the wood;

A more specific object ofthe. invention is to provideamethod of. impregnating wood by which the poresof the wood, may be impregnated substantially throughout the entire extent of? the body of the. wood, leavingvno space within the body for moisture.

Another object of the invention is to provide a method of wood impregnation which may be eco-. nomically effected without any substantial splitting or checking of the Wood. even though the wood to be treated. contains substantialiquantities' of water prior to impregnation.

A further objectof'the invention istoprovide a method of wood impregnation in which water is removed from ablank of regulargeometryywithout deleterious action .on the blank whichwould militate against subsequent. fabrication operations and impregnationis effectedeither subsequent to fabrication or immediately after, water removal.

An additional objectof thev invention isgto pro: vide a method of impregnation which will not, re-. quire the usual pressure. or vacuumsystemshut which will prcvidefor substantially complete im: pregnation of the wood.

A still further object of the invention isfito. pro-.- vide a method by which substantially complete impregnation at atmospheric pressure may, be achieved in a relatively short periodof. time.

One of the .most. common methods. of; impregq hating wood is to. immerse. air or. kilnrdried wood in a suitable impregnatingbathdisposedwithin acontainer and then apply pressure to thebath and. the wood in the'containerv to. force the. im-. pregnant into the pores of thewood. Thisisa relatively slow process and onewhich is not al: ways; efiective for securing. sufficiently, complete impregnation to insure. against dimensional change in the impregnatedwoodand eliminate the possibilityof moisture gainwhich is. objectionable, as mentioned above. Vacuum processes of wood impregnation.havealsobeen used aswell as combination pressure. and vacuum processes, performed, as, alternate pressure. application and. vacuum creation. These, processes have notbeen completely successful in accomplishing full ims pregnation and they are, ofcourse, relatively expensive, both in the costof necessary equipment and in operating expense. I

According to the present. invention, the; wood tov be impregnated is first subjected to ahigh ire:- quencyelectrical field to heat the wood bythe dielectric efiect to removemoisturefrom the wood It until it has an essentially zero water content, and the temperature of the wood is then elevated to a degree above the boiling point of Water but below the charring temperature of the wood. By heating the wood dielectrically, two important advantages are derived over conventional drying methods. First, it is possible to rapidly completely dry the wood by a substantially uniform generation of heat at an extremely low temperature, compared with normal commercial practice, and thus eliminate end grain checking and cracking, even though the wood initially contains an extremely large amount of Water. Second, by dielectric heating it is possible to deliver the wood to the impregnating bath at a substantially uniform temperature throughout; whereas, with ordinary drying methods, the outer portions of the wood are heated to a higher degree than the inner portions. By having the interior of the wood at least as high as the exterior and possibly somewhat higher due to heat losses from the surfaces during and subsequent to drying, an unusually effective vacuum action is obtained without evacuating equipment when, as a subsequent step in the process of the present invention, the heated wood is immersed at normal atmospheric pressure in an impregnating bath the temperature of which is substantially lower than the temperature of the wood. In the attached drawing:

Figure l is a flow diagram indicating a preferred sequence of steps to be performed in carrying out the method of this invention; Figure, 2 is a perspective view of a Wood blank of the type used in, the fabrication of a gear tooth;

Figure 3 is a perspective view illustrating a partially fabricated gear tooth; and

Figure 4 is a perspective view illustrating a completed gear tooth, ready for assembly in a supporting metal frame.

Appropriate notations have been applied to the various items shown in the drawing. l" A method of impregnating a blank for the formation of a gear tooth will be described as typical of the present invention, but it will be understood that the invention is not limited to any particular size or shape of blank or article, the method being one of general application. In themanufacture of a gear tooth, a blank of air or kiln-dried hard maple wood may be used, although relatively green wood containing a largequantity of moisture may be treated, if desired. Air-dried or kiln-dried wood normally contains in the order of of water and green wood may contain as much as or more of water. Preferably, the blank is of regular gebmetry, such as a rectangular piece in which the length, width, and thickness are each uniform throughout. This facilitates uniform heating for water removal, as will be more fully discussed. A gear tooth blank 2" thick by '7" wide by 6" long will be considered typical.

The blank is placed between electrodes connected to a source of high frequency power such as a" conventional electronic oscillator delivering a frequency of 10 'megacycles and a voltage of about 1 kilovolt per inch of thickness of the blank disposed between the electrodes. The power applied is so adjusted that the temperature of the blank is elevated to about 210 F. There is a dissipation of vapor from within the wood as this heating continues. Heat is generated substantially uniformly throughout the extent of the blank by subjecting theblank to the high frequency field; and, as a result, rapid dissipation of water from the wood is effected at a low temperature as compared with usual wood drying where superheated steam is generally used and where, due to the combined effect of the nonuniformity of the heating and the high temperature application, objectionable edge checking and cracking occur and complete water removal is difficult to achieve. Water removal should be accomplished as soon after the blank has been formed as possible, particularly where green wood is used, for there is a tendency for the wood to end check after cutting where moisture is permitted to leave the blank adjacent the freshly cut surfaces.

Usually about five to ten minutes time is required to remove substantially all the moisture from within a blank of the size mentioned utilizing the electronic oscillator referred to. In some instances, longer periods of time may be required depending upon the type of wood, the size of the blank, its moisture content, the initial tem perature of the blank, and other variable factors. The application of the high frequency field to the blank is continued until the water content has been reduced to essentially zero. There may be some water chemically bound in the cellulosic structure of the material, but reference is made to the water disposed within the mass, essentially within and between the fibers, as the term water content is generally recognized in the lumber industry. By essentially zero there is contemplated a moisture content of not more than 2% and generallyless than 1% and more frequently a bone dry condition, insofar as this is physically possible.

After the wood has had its water content reduced to essentially zero, the dielectrical heating is preferably continued at a higher voltage, 2 kilovolts per inch of thickness in the blank under consideration, and the temperature of the wood is raised above the boiling point of water. The temperature to which the wood is elevated will depend to a large extent upon the particular saturant to be employed as well as the type of wood, the size of the blank, and other variable factors. The wood temperature should in all events be higher than the temperature of the impregnant and should preferably be at as high a temperature as can be safely and economically attained. As a general rule, it may be stated that the working range for the temperature of the wood will lie between 212 F., the boiling point of water, and the charring temperature for the particular wood employed. The range normally used will be between 250 F. and 350 F. and optimum results will be obtained with most woods when the temperature is between 275 F. and 325 F. In the specific example under consideration, the temperature of the maple blank may be raised to 300 F.

The frequency of the source ofhigh frequency power will vary depending primarily upon the geometry of the load and the heating rate desired. Frequencies between approximately .5 megacycle and 40 megacycles' are preferred. The voltage gradient or watt density used in heating the wood will be dependent upon the type of wood, open or tight grain, and the grain length, as well as other variable factors. The rate of heating should be such that the development of excessive pressures and explosions of the wood fiber are avoided. As mentioned above, gear teeth blanks of hard maple about 2" x '7" x 6" were heated successfully with a voltage gradient of -1 emanates kilovolt 'per inch of thickness during prying,

elevated to 2 kilovolts per inch after drying.

The wood blank substantially completely' freed of water and while at anelevated temperature is immersedin an impregnating bath which is 'to fill thepores of the wood and thus prevent the wood from taking "on any -moisture. "Where thermoplastic impregnants are 'used, the temperature of the impregnating bath should, of course, be suihciently high to melt the impregnant and render it sufiiciently fluid that it'will 'be capable of penetrating the wood to the "desired extent. Where liquid impregnants are used they may be at room temperature. The general rule is to have the temperature of the saturant as low as possible and the wood temperatureashigh as possible, both consistent with limitations imposed by safety'and sound'commercial'practice, for the utmost advantage in rapid and complete penetration of the impregnant is achieved when the temperature diiierential between the wood and the impregnant is at itsmaximum. A 'tem perature difierential of at least 50 F. should exist for economical commercial practice-with most woods and usual impregnants.

By having the wood at a higher temperature thanthe temperature of the impregnating-bath, there is a vacuum action created upon thecooling of the wood within the bath, and since the temperature of the interior of the-wood is'at least impregnating bath. Ceresin Wax having'a melting point of 160 F. heated to .170 F. will be.

found to rapidly andefiectively penetratea-maple blank of the dimensions given above, heated to 300 F. at the time of immersion, in a period of thirty to sixty minutes, during which time the 'wood will absorb 25% of its weight of the ceresin wax. The quantity absorbed .will vary with different woods as well as diflerent impregnants. Generally 15% by weight based on the weight of the dry wood will'be a minimum for close-grained woods.

Subsequent to impregnation, the blanksv are'removedfrom the impregnatingbath, wiped'free of excess impregnant, and are then ready for fabrication.

Observations which have been made of wood blanks treated in accordance with the foregoing methodyas well as dielectric measurements which have been taken, show no measurable absorption of water by the treated pieces, and no measurable dimensional changes have occurred in articles:

fabricated from blanks treated in accordance with the invention.

In the manufacture of a mold wall for the dielectric heating of cork compositions, blanks -01! maple wood 3 thick by'll wide by 18"long were treated. The high frequency oscillator delivered a frequency of 10 megacycles andtheini- -tial voltage gradient or watt density was .5 kilvolt ..per inch of thickness which was continued until -;dryin g was effected ;and :was 1. increased :to :g

'6 1 -kilovolt -per inch :of thickness for the :subse quent' heating to about"300'Z-F. :sh-rinkage "or of an inch in an "ll-inch iwidthpracross "the grain directionwas noted, and-a slight war-page resulting from stress relief in the blank was "observed, approximating a inch displacement in the-center of the -11 inchwidth. No substantial checking was noted and the 'Warpage was,

of course, so slight that it was merely necessary toslightly increase the nominal dimensions oftthe blank I prior to "treatment The blanks were approximately A inch "greater than the -over a-ll dimensions of the final panels. If minorwarpage or "the work piece-during -water removal is ::;a factor of 1 any considerable importance, the :work

piece may be placed underpressure between lthe electrodes "'during the drying process. 'ThlSWVill tend to reduce or eliminate all warpagewalthough generally -=there "will "be: some 1 minor deformation .of the blank upon removal from ithe press due largely.- to the release of stresses-Withthe body of the blank.

It is preferredto dielectrically heat -wood blanks of regular geometry (such a blank-has been illustrated in Figure 2), for this -makes possible the use of conventional dielectric "heating equipment without any special precautions to :prevent "undesired overheating in certain areas 'of the blank which might occur if thepieces'were not of the regular contour and such irregularity was *not taken into account in the design of Jthe electrodes. Irregularly -shaped *blanks f may the treated in accordance with the invention, however, and it is #also within -the'scope ofthe invention to treat finished or partially completed articles. 'i-Where the cost of the impre an't -a major factor, 'ith'as "been found=-possible1:to

practice the invention in this: order: formztthe blank, subject the blank -tothe high frequency electrical field to reduce the water content'to essentially zero, then 'fabricat'e the blank to :an

article of substantially the desired finalishape. (as

shown in Figure?) ,.:reheat the blank, preferably by the dielectric 'aeffect, to. a Ltemperature 'rabtove the temperature of the impregnant but :below the -charring itemperature rof :the :wood,;iand .ithen impregnate. Any 'ifinal finishing zoperations which -may be=necessary canthen :be efiected. Figure 4 shows a completed geartooth tfinished from the fabricated blank-of Figure"2. Where minonwarpage is not objectionable or can-beconveniently obviated by pressure application, the

"process may be carried outon completelyfinished articles.

While it is preferred to utilize the dielectric eiiect for elevatingthetemperature of the blank to the desired degreesubsequent to drying; this is not "essential, for a blank which has been di- In most inbath.

The invention is not limitedato any-particular wood but :is applicable: to fibrous materials of various sorts in which the attainment of v.di-

mensional stability is a problem and :is particularly useful where itis'necessaryxto secnresubstantially complete filling of the voids -within ;the "fibrous 1 material by an impregnating agent, .asin the case of mold walls'for.dielectridheating where, aszmentionedabovegthe:presence.o'f moisture affects the rate of dielectric heating and the voltage insulating value of the mold wall.

While in the typical example reference has been made to ceresin wax as an impregnant, other materials may be used. Where complete filling of the void space is desired, a melted impregnantis generally preferred, but for other uses solvent types of impregnants may be substituted. Synthetic resinous impregnants, such as phenol aldehyde resin in which water or alcohol may be employed as the solvent, depending upon the stage of polymerization of the resin, are typical of the solvent type of impregnants. With this latter type of impregnant, it may be maintained at about room temperature at the time of immersion of the blanks to be treated Generally it is desirable to impregnate the whole of the article, but with the present invention it is possible to localize the heating and impregnate only in those areas which have been heated. For example, local impregnation of end sections of wood is frequently desired, and this maybe economically accomplished by the method of this invention.

While I have illustrated and described certain preferred embodiments of my invention, it will be understood the same is not limited thereto but may be otherwise embodied and practiced within the scope of the following claims.

I claim:

1. In a method of impregnating wood with an impregnant, the steps comprising subjecting the wood to a high frequency electrical field to heat the wood by the dielectric effect to a temperature below about 212 F. until its water content has been reduced to essentially zero, heating said wood to a temperature substantially above 212 F. and immersing said wood heated to a temperature substantially above 212 F. and at essentially zero content of water in a bath of impregnating material the temperature of which is at least 50 F. lower than the temperature of the wood.

2. In a method of impregnating wood with a thermoplastic material, the steps comprising subjecting the wood to a high frequency electrical field to heat the wood by the dielectric effect to a temperature below about 212 F. until its water (content has been reduced to essentially zero,

heating the wood to a temperature substantially above 212 F. and below its charring temperature, and immersing said wood while so heated in a bath of thermoplastic impregnant heated to a temperature above the melting point of the impregnant but at least 50 F. lower than the temperature of the wood.

3. In a method of impregnating wood with a thermoplastic material, the steps comprising subjecting the wood to a high frequency electrical field to heat the wood by the dielectric effect to a temperature below about 212 F. until the water content of the wood has been reduced to essentially zero, thereafter raising the temperature of the wood to between 250 F. and 350 F., and immersing said wood while so heated in a melted bath of thermoplastic impregnating material heated to a temperature above its melting point but below 250 F.

4. In a method of impregnating wood with a [thermoplastic material, the steps comprising subjecting the wood to a high frequency electrical field to heat the wood by the dielectric effect to a temperature below about 212 F. until its moisture content has been reduced to essentially zero, heating said wood to a temperature of about 300 F., and immersing said wood heated to atemperature of about 300 F. in a bath of melted waxlike material heated to a temperature of about F.

5. In a method of impregnating maple wood with a thermoplastic material, the steps comprising subjecting the wood to a high frequency electrical field to heat the wood by the dielectric effect to a temperature of about 210 F. until its moisture content has been reduced to essentially zero, thereafter heating the wood to a temperature between 275 F. and 325 F., immersing said wood while so heated and while at a substantially zero moisture content in a melted bath of impregnating material at a temperature substantially below the temperature of the wood, and maintaining said wood in said impregnating bath until at least 25% by weight of impregnant based on the weight of the dry wood has been disposed within the body of the wood.

6. In a method of impregnating wood, the steps comprising subjecting the wood to a high fre quency electrical field to heat the wood by the dielectric effect throughout its extent to a temperature about the boiling point of water until its moisture content has been reduced to essentially zero, subjecting the wood to the further action of a high frequency electrical field to elevate the temperature of the wood throughout to a degree above 212 F. and below the charring temperature of the wood, with the interior portion of the wood at a temperature at least as high as the temperature of the surface portions, and immersing said wood while so heated into a fluid bath of impregnating material the temperature of which is below 212 F.

'7. In a method of completely filling wood throughout its body with an impregnant to eliminate the absorption of moisture by the impregnated wood, the steps comprising subjecting the wood to a high frequency electrical field in two stages, first to heat the wood to a temperature about the boiling point of water where water will be driven therefrom until the water content of the wood has been reduced to essentially zero; and, second, to heat the wood to a temperature substantially above 212 F. but below its charring temperature after substantially all of the water has been removed from the wood to thus produce a heated body of wood free of end checking or cracking the temperature of which is higher within the body than at the surfaces thereof, and applying a fluid impregnant to said body at a temperature which is substantially below the temperature of the wood, whereby the wood upon cooling will cause the impregnant to be drawn into the body thereof to completely fill the same therethroughout.

8. In a method of impregnating wood, the steps comprising subjecting the wood to a high frequency electrical field to heat the wood by the dielectric effect to a temperature of about 212 F. where moisture will be driven from the wood until the water content of the wood has been reduced to essentially zero, subjecting the wood to a further action of a high frequency electrical field to heat the wood to a temperature between 270 F. and the charring temperature of the wood, and immersing the wood while so heated into a bath of impregnating material the temperature of which is below 212 F.

9. In a method of impregnating wood, the steps comprising confining said wood between pressing surfaces, subjecting the wood to a high frequency electrical field in two stages, first to heat the wood by the dielectric effect to a temperature about the boiling point of water until its moisture content has been reduced to essentially zero, and second to heat the wood to a temperature between 212 F. and its charring temperature, the second-stage heating being effected by subjecting the Wood to a highi'requency field at a higher voltage than in the first-stage heating, and applying to the wood while so heated a melted impregnant the temperature of which is lower than the temperature of the wood.

10. In a method of substantially completely filling Wood throughout its body with a waxlike impregnant to eliminate the absorption of moisture by the impregnated wood, the steps comprising subjecting the wood to a high-frequency electrical field in two stages, first to heat the wood to a temperature of about 210 F., at which water will be driven from the wood, until the water content reaches essentially zero without cracking or checking of the wood, and second to heat the wood to a temperature between about 275 F. and about 325 F after substantially all of the water has been removed from the wood REFERENCES CKTED 15 The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 20 Re.5,04-O Palmer Aug. 2%), 1872 67,104 Clarke July 23, 1867 686,582 Brinkerhofi" Nov. 12, 1901 707,224 Giussani Aug. 19, 1902 2,397,615 Mittelmann Apr. 2, 1946 

1. IN A METHOD OF IMPREGNATING WOOD WITH AN IMPREGNANT, THE STEPS COMPRISING SUBJECTING THE WOOD TO A HIGH FREQUENCY ELECTRICAL FIELD TO HEAT THE WOOD BY THE DIELECTRIC EFFECT TO A TEMPERATURE BELOW ABOUT 212* F. UNTIL ITS WATER CONTENT HAS BEEN REDUCED TO ESSENTIALLY ZERO, HEATING SAID WOOD TO A TEMPERATURE SUBSTANTIALLY ABOVE 212* F. AND IMMERSING SAID WOOD HEATED TO A TEMPERATURE SUBSTANTIALLY ABOVE 212* F. AND AT ESSENTIALLY ZERO CONTENT OF WATER IN A BATH OF IMPREGNATING MATERIAL THE TEMPERATURE OF WHICH IS AT LEAST 50* F. LOWER THAN THE TEMPERATURE OF THE WOOD. 